Diagnosis of monochorionic twin pregnancy
How is monochorionicity diagnosed prenatally and what is the accuracy of prenatal ultrasound
chorionicity allocation?
All women with a twin pregnancy should be offered an ultrasound examination between
11+0 weeks and 13+6 weeks of gestation (crown–rump length 45–84 mm) to assess fetal viability,
gestational age and chorionicity, and to exclude major congenital malformations.
B
Chorionicity should be determined at the time the twin pregnancy is detected by ultrasound based
upon the number of placental masses, the appearance of the membrane attachment to the
placenta and the membrane thickness. This scan is best performed before 14 weeks of gestation.
[New 2016]
D
A photographic (thermal copy) record should be taken and placed in the patient’s notes
documenting the ultrasound appearance of the membrane attachment to the placenta and an
electronic copy stored (Appendix II).

If there is uncertainty about the diagnosis of chorionicity, a photographic record of the ultrasound
appearance of the membrane attachment to the placenta should be retained and a second opinion
should be sought.

If there is still doubt in the diagnosis of chorionicity, the woman should be referred to a specialist
without delay, as chorionicity is best determined before 14 weeks of gestation. D
On ultrasound, the fetuses in twin pregnancies should be assigned nomenclature (i.e. upper and
lower, or left and right) and this should be clearly documented in the woman’s case notes to ensure
consistency throughout pregnancy.

Outcome of monochorionic twin pregnancy
What is the outcome of monochorionic compared with dichorionic twin pregnancies?
Clinicians and women should be aware that monochorionic twin pregnancies have higher fetal loss
rates than dichorionic twin pregnancies, mainly due to second trimester loss and, overall, may have
a higher risk of associated neurodevelopmental morbidity. This should form part of the parental
counselling.

Optimal screening for chromosomal abnormalities, structural abnormalities and other fetal
complications in monochorionic twin pregnancies
What is the optimum method of screening for chromosomal abnormalities in monochorionic twin pregnancies?
Women with monochorionic twins who wish to have aneuploidy screening should be offered nuchal
translucency measurements in conjunction with first trimester serum markers (combined screening
test) at 11+0 weeks to 13+6 weeks of gestation (crown–rump length 45–84 mm). [New 2016]
C
In women with monochorionic twin pregnancies who ‘miss’ or who have unsuccessful first trimester
screening for aneuploidy, second trimester screening by the quadruple test should be offered.
[New 2016]
D
Early data with noninvasive prenatal testing are encouraging, but results should be interpreted with
caution until larger studies have been carried out. [New 2016] C
What is the optimum method of screening for structural abnormalities in monochorionic twin pregnancies?
All monochorionic twins should undergo a routine detailed ultrasound scan between 18 and
20+6 weeks of gestation which includes extended views of the fetal heart anatomy (as
recommended in the Fetal Anomaly Screening Programme screening of a singleton fetus).
C
What is the optimum ultrasound regimen for monochorionic twin pregnancies?
Fetal ultrasound assessment should take place every 2 weeks in uncomplicated monochorionic
pregnancies from 16+0 weeks onwards until delivery (Appendix III). D
At every ultrasound examination, liquor volume in each of the amniotic sacs should be assessed
and a deepest vertical pocket (DVP) depth measured and recorded, as well as the umbilical artery
pulsatility index (UAPI). Fetal bladders should also be visualised. Although first presentation of
twin-to-twin transfusion syndrome (TTTS) is rare after 26+0 weeks of gestation, it can occur and
therefore, scans should be performed at 2-weekly intervals in uncomplicated monochorionic twins
until delivery (Appendix III). 

From 16+0 weeks of gestation, fetal biometry should be used to calculate an estimated fetal weight (EFW) and the difference in EFW calculated and documented. As the risk of selective growth restriction (sGR) extends to delivery, this should be performed at 2-weekly intervals until delivery.

What are the optimum methods of screening for specific complications of monochorionic twin pregnancies?
Screening for TTTS
Screening for TTTS by first trimester nuchal translucency measurements should not be offered.
[New 2016] C
Women with monochorionic twin pregnancies should be asked to report sudden increases in
abdominal size or breathlessness to healthcare professionals in their secondary or tertiary centres as
this may be a manifestation of TTTS.

Screening for TTTS should be by ultrasound examination from 16+0 weeks onwards, at 2-weekly
intervals, noting and recording fetal biometry and liquor volumes (DVP). Fetal bladders should also
be visualised.

Screening for twin anaemia-polycythaemia sequence (TAPS)
TAPS should be screened for following fetoscopic laser ablation for TTTS and in other complicated
monochorionic pregnancies requiring referral to a fetal medicine centre (such as those complicated
by sGR) by serial middle cerebral artery peak systolic velocity (MCA PSV). [New 2016]

Screening for sGR
At each scan from 20 weeks of gestation (at 2-weekly intervals) onwards, calculate EFW discordance
using two or more biometric parameters. Calculate percentage EFW discordance using the following
formula: ([larger twin EFW – smaller twin EFW]/larger twin EFW) x 100. Liquor volumes as DVP should
be measured and recorded (to differentiate from TTTS). [New 2016]
C
An EFW discordance of more than 20% is associated with an increase in perinatal risk. Such
pregnancies should be referred for assessment and management in fetal medicine units with
recognised relevant expertise. [New 2016]
B
Umbilical artery Doppler evaluation in monochorionic twins with sGR allows definition of prognosis
and potential morbidity. In particular, those with absent or reversed end-diastolic velocities (AREDV)
and ‘cyclical’ umbilical artery Doppler waveforms (intermittent AREDV) are at increased risk of
perinatal mortality and morbidity (Appendix IV). [New 2016]
C
Management of complex pathologies associated with a monochorionic twin pregnancy
The management of TTTS
How useful are grading systems for severity of TTTS in establishing prognosis?
At diagnosis, TTTS should be staged using the Quintero system. In addition, measurement of
umbilical artery Doppler velocities, MCA PSV and ductus venosus Doppler studies should be
performed and documente.

What is (are) the optimal treatment(s) of TTTS and their outcomes?
TTTS should be managed in conjunction with fetal medicine centres with recourse to specialist
expertise and treatment in supraregional centres. 
TTTS presenting before 26 weeks of gestation should be treated by fetoscopic laser ablation rather
than amnioreduction or septostomy. There is evidence that the fetoscopic laser ablative method
should be the Solomon technique.
A
Centres performing fetoscopic laser ablation should perform at least 15 procedures per year (rolling
3-year average). [New 2016] 
Weekly ultrasound assessment (including examination of the fetal brain, heart and limbs) and serial
measurements of UAPI, MCA PSV and ductus venosus Doppler velocities should be performed. After
2 weeks post treatment, the ultrasound interval can be increased to every 2 weeks (noting UAPI,
MCA PSV and DVP) with documentation of adequate fetal growth (by calculating EFW).

In treated TTTS pregnancies, ultrasound examination of the fetal heart should be performed by the
fetal medicine specialist to exclude functional heart anomalies. 
When should the delivery of monochorionic twin pregnancies complicated by TTTS take place?
Delivery of monochorionic twin pregnancies previously complicated by TTTS and treated should be
between 34+0 and 36+6 weeks of gestation. [New 2016] D
The management of sGR
sGR in monochorionic twins requires evaluation in a fetal medicine centre with expertise in the
management of such pregnancies. [New 2016] 
In cases of early-onset sGR in association with poor fetal growth velocity and abnormal umbilical
artery Doppler assessments, selective reduction may be considered an option. [New 2016] C
In sGR, surveillance of fetal growth should be undertaken at least every 2 weeks with fetal Doppler
assessment (by umbilical artery and middle cerebral artery pulsatility index, and peak systolic
velocity). If umbilical artery Doppler velocities are abnormal, the Doppler assessments should be
undertaken in line with national guidance, measuring ductus venosus waveforms. [New 2016]
D
Clinicians should be aware that there is a longer ‘latency period’ between diagnosis and delivery in
monochorionic twins complicated by sGR compared with growth restriction in dichorionic twin
pregnancy or singleton pregnancy. [New 2016]
D
Abnormal ductus venosus Doppler waveforms (reversed flow during atrial contraction) or
computerised cardiotocography short-term variation should trigger consideration of delivery.

In type I sGR, planned delivery should be considered by 34–36 weeks of gestation if there is
satisfactory fetal growth velocity and normal umbilical artery Doppler waveforms. [New 2016] 
In type II and III sGR, delivery should be planned by 32 weeks of gestation, unless fetal growth
velocity is significantly abnormal or there is worsening of the fetal Doppler assessment. [New 2016] 
It is important to prospectively inform parents that in sGR and TTTS (even after apparently
successful treatment) there can be acute transfusional events (which are neither predictable nor
preventable) and therefore, despite regular monitoring, there may still be adverse perinatal
outcomes. [New 2016]

Management of TAPS
Clinicians should be aware that the natural history, fetal and neonatal implications, and optimal
treatment and/or surveillance of monochorionic pregnancies diagnosed with TAPS are poorly
established. [New 2016]
D
The management of monochorionic twin pregnancies complicated by single twin demise
What are the consequences for the surviving twin after fetal death of the co-twin in a monochorionic pregnancy and
what is optimal clinical management?
Clinicians should be aware that monochorionic pregnancies not complicated by TTTS, sGR or TAPS
are still at risk of fetal death and neurological abnormality. D
After a single fetal death in a monochorionic pregnancy, clinicians should be aware that the risks to
the surviving twin of death or neurological abnormality are of the order of 15% and 26%,
respectively. [New 2016]
B
Single fetal death in a monochorionic pregnancy should be referred and assessed in a fetal medicine
centre, with multidisciplinary expertise to manage these cases. 
Fetal magnetic resonance imaging of the brain may be performed 4 weeks after co-twin demise to
detect neurological morbidity if this information would be of value in planning management. D
How should fetal anaemia be monitored after single twin intrauterine death?
Fetal anaemia may be assessed by measurement of the fetal MCA PSV using Doppler
ultrasonography.

Timing and mode of delivery in uncomplicated monochorionic pregnancies
What is the optimal timing and method of delivery for otherwise uncomplicated monochorionic pregnancies (without
TTTS, sGR or TAPS)?
Women with monochorionic twins should have timing of birth discussed and be offered elective
delivery from 36+0 weeks with the administration of antenatal steroids, unless there is an indication
to deliver earlier. [New 2016]
C
It is appropriate to aim for vaginal birth of monochorionic diamniotic twins unless there are other
specific clinical indications for caesarean section. A
Monochorionic monoamniotic (MCMA) pregnancies
What are the specific problems of MCMA pregnancies and how should they be managed?
MCMA twins almost always have umbilical cord entanglement when visualised using colour flow
Doppler. Such a finding has not consistently been demonstrated to contribute to overall morbidity
and mortality. [New 2016]
D
MCMA twins have a high risk of fetal death and should be delivered by caesarean section between
32+0 and 34+0 weeks. [New 2016] D
Higher order multiple pregnancies with reference to monochorionicity
What is the outcome of monochorionic and dichorionic compared with trichorionic triplet pregnancies?
Clinicians should be aware that monochorionic/dichorionic triplet pregnancies have higher fetal loss
rates than trichorionic triplet pregnancies and may be complicated by feto–fetal transfusion
syndrome, sGR and TAPS.
C
Selective reduction should be discussed in all higher order pregnancies including triplets. 

Increased ultrasound surveillance is warranted in a fetal medicine centre with expertise to manage
such cases. 
Discordant abnormalities in monochorionic pregnancies
What is the incidence of and the therapeutic options for discordant abnormalities in monochorionic pregnancies,
including twin reversed arterial perfusion sequence?
Monochorionic twins that are discordant for fetal anomaly must be referred promptly for
assessment and counselling in a fetal medicine centre with consideration for treatment. 
Karyotyping of monochorionic twins should be managed in a fetal medicine centre.

Meticulous mapping of the position of the twins within the uterus should be performed both at the
time of prenatal diagnostic tests and invasive treatments. [New 2016] 
During amniocentesis, both amniotic sacs should be sampled in monochorionic twin pregnancies,
unless monochorionicity is confirmed before 14 weeks and the fetuses appear concordant for
growth and anatomy. [New 2016]

Prior to invasive testing or in the context of twins discordant for an abnormality, selective reduction
should be discussed and made available to those requesting the procedure after appropriate
counselling. [New 2016]

Monitoring for disseminated intravascular coagulopathy is not indicated in monochorionic twin
pregnancies undergoing selective reduction. [New 2016] 
Selective feticide by intravascular injection of an abortifacient is not an option in monochorionic
pregnancies because of the presence of placental anastomoses. The potential risks of intrafetal/
umbilical cord ablative procedures should be discussed prospectively, including the risk of co-twin
loss and neurological morbidity. [New 2016]

Documentation and discussion of heterokaryotypic monozygotic karyotypic abnormalities should take
place. [New 2016] 
Conjoined twins
How are conjoined twins diagnosed and what are the outcomes?
Conjoined twins are exceedingly rare and prenatal assessment is required in a tertiary fetal
medicine centre so that diagnosis can be confirmed and prognosis discussed in conjunction with a
multidisciplinary team. [New 2016]

What are the training competencies required for managing monochorionic pregnancies?
All sonographers who undertake routine ultrasound scans during pregnancy must be trained to
establish chorionicity and the correct labelling of twins. 
All sonographers who undertake midtrimester (18+0–20+6 weeks) and fetal growth scans of
monochorionic twins should be made aware of the appearances of TTTS, sGR and TAPS, and the
need to refer patients on to specialist centres if such features present.

Fetal medicine centres undertaking fetal therapy for relatively rare complications of monochorionic
twins should have a minimum of two experienced operators and more than 15 cases per year
(rolling 3-year average) to maximise perinatal outcomes and minimise long-term morbidity.